1. Field of the Invention
The present invention relates to a solid state laser pumped by laser diodes.
2. Discussion of the Art
A laser diode pumped solid state laser produces laser light by pumping a resonator of a laser material, such as Nd:YAG or Nd:YLF, with light from laser diodes, exciting atoms or molecules within the laser material. The ends of the laser material may be polished and covered with a highly reflective coating, or external reflectors may be used, to form an oscillator cavity, reflecting light back and fourth through the laser material. This provides the feedback for optical amplification, resulting in a beam of laser light. These types of solid state lasers are well known to those of ordinary skill in the art, and are described in "Lasers", Encyclopedia of Chemical Technology, Kirk-Othmer, 4.sup.th ed., vol 15, pp. 1-50, (John Wiley and Sons, 1995) and "Lasers (Coherent Sources)", J. E. Miller and D. J. Horowitz, Electronics Engineers' Handbook, 3.sup.rd Edition, D. G. Find and D. Christiansen, ed., pp. 11-11 to 11-41, (McGraw-Hill, 1989).
Laser diodes emit a light beam with an elliptical shape. A laser diode array is illustrated in FIG. 17. A laser diode array 2 contains many individual laser diodes; as many as 100 is not unusual. Each diode emits its own laser beam 42 from the emitting surface 40, which together form an elliptical light beam 32. The laser beam 32 fans out more rapidly in the vertical direction 44, typically 20-50.degree., but only 8-15.degree. in the horizontal direction 46. Since only light which impinges on the laser material (resonator) can excite the atoms or molecules in the laser material, the laser diode array is placed as close as possible to the laser material, in order to avoid losing any light.
FIGS. 1 and 2 illustrate a conventional laser diode pumped solid state laser having a side (perpendicular) pumping configuration. The axis which is aligned with the resonator cavity, the laser axis, is the long axis of the resonator in FIG. 2, or the long axis perpendicular to the direction of illumination by the laser diode arrays. Omitted from this figure (as well as most of the figures) are end mirrors and optional Q-switches which define the resonator cavity. In FIG. 1 a slab shaped resonator is pumped by two semiconducting laser diode arrays 2 and 2. Antireflective coatings 28 and 28, on the surface of the resonator facing the laser diode arrays prevents loss by reflection of the pumping laser light. As the light passes through resonator 12 it excites atoms or molecules in the laser material. Highly reflective coatings 14 and 14 then reflect any light which is not absorbed back through the resonator 12.
FIG. 2A illustrates a rod shaped resonator 6 pumped by a laser diode array 2. An exploded view of two alternative configurations are shown in FIGS. 2B and 2C. In a first configuration, the surface of the resonator 6 is coated with an antireflective coating 28. The laser beam 32 generated by the laser diode array 2 passes into the resonator 6 and is reflected back by the highly reflective coating 14. In FIG. 2C the entire resonator 6 is covered by an antireflective coating 28, and the laser beam 32 is reflected back after it exits the resonator by a concave mirror 30.
The light from the laser diode arrays is not completely absorbed through the thickness of the laser material. To most efficiently use the light from the laser diode arrays, it is necessary to reflect the laser diode light which escapes the laser material opposite the laser diode arrays. Since the beam of light from the laser diode arrays fan out, reflection of this light from a flat mirror results in a large amount of this reflected light not being returned to the resonator. Therefore, the light must be redirected, typically with a concave mirror, which refocuses the light back towards the resonator. If the resonator is in the shape of a round cylinder, this may also be accomplished by coating the surface of the resonator opposite the laser diodes with a highly reflective coating.
The close proximity of the laser diode array to the resonator is problematic. Only a small portion of the resonator is actually illuminated, and hence excited, by the light from the laser diode array. Consequently, only a small fraction of the resonator participates in formation of the laser beam. Furthermore, the heat generated by the laser diode arrays locally heats a portion of the resonator, especially when the laser output power is 10 watts or more. The localized heating degrades beam quality, for example by thermal lensing, resulting in a beam with a varying pointing direction, unstable energy profile and variable spot size. In addition, the ends of the resonator are typically polished and coated with an antireflective coating, and thermal effects can distort the flatness of this polished end, causing it to act like a lense, changing the optical characteristics of the resonator cavity.
The thermal distortion effects in laser diode pumped solid state lasers necessitate complex cooling systems. For example, closed loop water cooling is a requirement in laser diode pumped solid state lasers of 10 watts or more. Water cooling is problematic because of its complexity, and the necessity of adding water to the cooling system. Water cooling will only efficiently cool a portion of the resonator, unless the entire resonator is immersed in the cooling water. Total immersion, however, can interfere with the resonator cavity, requiring very stringent water conditions. Finally, there is also the possibility of leakages, which can result in short circuits and destruction of the entire laser system and any accompanying electronic equipment, making a water cooled system unsuitable for use in printers and other consumer devices.